170 Proceedings of Royal Society of Edinburgh. [sess. 
symptomatology and description of the microscopic examination of 
the nucleus.) 
A considerable body of evidence has thus accumulated in favour 
of an anatomical segmentation of the nucleus in question, but no 
two authors seem to be agreed on the position and number of the 
segmented groups. To investigate the point further, I made a series 
of transverse vertical sections of the brain of a human foetus, from 
the lower border of the fourth to the upper margin of the third 
nucleus ; and a second series, from an adult human brain, of 
longitudinal transverse sections through the same nuclei. The first 
series is the more instructive. The divisions of the nuclei are 
represented in figures 1-8 ; one of the longitudinal transverse 
sections is shown in figure 9. From these and the special descrip- 
tion of the plates which follows, it will be seen that the nucleus of 
the nerve to the superior oblique muscle which lies below the most 
inferior part of the nucleus of the third nerve bears the same rela- 
tion to the posterior longitudinal fasciculus as does the latter nucleus, 
but is always to be distinguished from it by the direction taken by 
its root fibres, which pass outwards and backwards along the border 
of the central grey matter and the formatio reticularis, while the 
rootlets of the third nerve always arise from its anterior (ventral) 
surface, and pass at once directly forwards. 
Coming to the nucleus under special consideration, we distinguish 
the following groups : — 
A. An anterior group. 
B. A postero-external group. 
C. A median nucleus. 
D. A postero-internal nucleus. 
E. A superior nucleus. 
A. The anterior or ventral group, which extends along the greater 
part of the nucleus (being absent only at its extreme upper termina- 
tion), lies in relation to the innermost part of the main body of the 
posterior longitudinal fasciculus, and bears the same relation to it 
throughout. It is distinctly segmented, and may be divided into 
an inferior nucleus (fig. 2, III. i.n.), characterised by the absence of 
commissural fibres between the two sides, and a main nucleus , 
which is again capable of being subdivided into at least three parts, 
